This invention relates to wind machines for converting wind energy to perform useful work, generally called windmills. Windmills extract power from a wind current by use of blades mounted to a rotatable structure to turn a shaft. The mass of a wind current impinging upon the blade, and flowing around it, transmits a force to the blade which is transformed into a torque about the drive shaft on which the blades rotate to power pumps, generators, compressors, or the like.
Various designs have been conceived for windmills. Some have used propeller-like blades connected to a hub which rotates on a horizontal shaft directed generally parallel to the wind flow. Machines of this type are commonly found on farms to power pumps, or the like, at remote locations. It is necessary to orient the orbiting blades perpendicular to the direction of the wind for this type of windmill, in order to properly expose them to the wind current so they will generate a rotational force. This directional sensitivity reduces the efficiency of these type windmills in any area having unstable gusting wind currents. It also requires that a steering mechanism be used to position the blades. Further, in recent years, structural difficulties have been found in making these machines of a sufficiently large size to produce the power necessary to meet current needs, especially for electricity generation. These difficulties arise not only from the type and height of a structure necessary to position the blades in an adequate wind flow, but also from the high centrifugal forces to which the rotating blades are subjected.
Other designs have a number of blades circularly mounted about a rotatable structure in carousel fashion. The structure includes a shaft positioned with its axis generally parallel to the axis of the blades, and perpendicularly positioned with the wind flow. These type machines, commonly referred to as cross-wind axis wind turbines, are usually installed with the blades and shaft positioned vertically with the ground surface. In this configuration the blade surfaces are exposed to wind currents blowing from any direction, making them capable of capturing energy with instantaneous response from directionally changing winds, without need of a steering device sensitive to wind direction. Further, due to the vertical position of the rotating shaft it is unnecessary to mount a driven implement or a right angle drive, such as a gear box, at a high elevation on the supporting structure of the windmill.
A blade of a wind machine obtains power from the wind by slowing the free stream wind speed downstream of the blade. In the design of windmills, or wind turbines, two principle motive forces can be generated from this wind speed change to provide torque about the rotating shaft. The first is a drag force acting on the blades which is caused by the wind current impinging on the surface of the blade. The drag force is created by the transfer of kinetic energy of the moving wind mass to the blade as the wind current is slowed by contacts with the surface of the blade as the wind flows around its form. Drag type wind machines are self-starting and generally produce high torque from their starting mode through low rotational speeds.
A drag-type wind machine, however, has inherent limitations. The tip speed of the rotating blade cannot be faster than the speed of the wind and usually it is somewhat less. This characteristic limits the rotational velocity of the shaft to which the blades are affixed, and it may require a transmission to obtain the shaft speed required for performing the desired work.
The ratio of the blade tip speed to the wind speed is commonly known as the tip speed ratio. This value is used as a measure of the functional range of efficient operation of the wind machine. Generally, a drag-type machine will produce optimum power when the tip speed of its blades approaches that of the free stream wind speed, meaning the tip speed ratio is close to one. However, a limit of the maximum tip speed attainable is also a limit to the amount of power which can be produced. A drag-type wind machine, being limited to a maximum ideal tip speed ratio of one, is thereby limited in its capability to produce power and in its efficiency. The maximum efficiency obtainable with the drag-type wind machine is a moderate value of about 30%, usually something less.
The second motive force employed to propel a wind machine is a lift force generated as wind current flows past an air foil. This type of wind machine uses a blade formed in the shape of an air foil positioned so that the lift forces generated by the wind current flowing over the blade will act in a direction to move the blade in its orbit. A component of the lift force in the direction of rotation is applied through a rotor structure to the rotating shaft to create a torque about the shaft.
Lift-type wind turbines are known to have blade velocities much higher than that of the free-stream wind speed. They therefore have tip speed ratios in excess of one, and often in the range of four to six. This is because blade speed is not directly dependent on a wind velocity component, but rather on a lift force component. Generally, the higher the tip speed ratio the more efficient the operation of the wind machine to produce power. The very high rotational speeds of lift-type wind machines adapt them for use with accessories that require high speeds, such as generators. The high rotational speeds also provide for a higher degree of efficiency and greater power production.
Generally, lift-type machine efficiencies are found in a range of 35 to 45%. Tip speed ratios may range from less than 1, as is common for the typical farm-type windmill, to between 4 and 6, as is common for vertical axis-type wind turbines as invented by G. J. M. Darrieus and described in U.S. Pat. No. 1,835,018.
The tip speed ratio is a critcal parameter of the lift-type machine, especially the Darrieus type wind turbines. Because a value curve of efficiency versus tip speed ratio for this type machine is highly peaked, a small change in the wind speed can result in a large change in efficiency, and a resultant loss of available power. This effect can be so severe as to cause the rotor to stop turning altogether. This so-called stall of the wind turbine may result from changes in the tip speed ratio due to wind gusts, i.e. an increase in wind velocity, as well as changes in the tip speed ratio from wind stagnation. Surmounting this characteristic normally requires a control system to vary the load placed on the turbine, or to vary the blade pitch angles more directly toward their relative wind flow, to prevent the machine from completely stopping.
Additionally, because of their narrow range of efficient operation, some common lift-type wind machine designs will not self-start, requiring a power input to the driven shaft to initiate rotation and bring the speed of the turbine up to a tip speed ratio of self-sufficient operation. This inability to begin rotation and accelerate to an efficient rotational speed is severe with the Darrieus lift-type (crosswind axis) wind turbine. It has given rise to auxiliary methods for self-starting which include the addition of external power sources apart from wind energy and the addition of drag-type blade forms mounted with the lift-type blades to the rotor structure to initiate rotation, as is described in the Bolie U.S. Pat. No. 4,204,805.
An increasingly common method of self-starting a Darrieus type wind turbine is the use of variable pitch blades on the rotor which are articulated to change their pitch angle with reference to the relative wind current as they travel around their carousel-shaped path. Blade articulation increases the total efficiency of the wind turbine by providing maximized lift force on the blades for a greater period throughout their orbital cycle. The blades are typically hinged on their longitudinal axis parallel to the axis of the driven rotating shaft so that they may be pivoted.
Various designs have been conceived for achieving pivotal movement of the turbine blades. Kinematic mechanisms have been used as was early shown in the Darrieus patent, and as is described in the Drees patent, U.S. Pat. No. 4,180,367. Drees further teaches the use of a gravitational force acting on a blade to pivot it perpendicularly to the wind between 90.degree. stops on one side of the turbine, while the turbine is at rest and at slow speeds. The wind creates a drag force on the perpendicularly positioned blade to begin and sustain rotation. Centrifugal force acting on the mass of the blade increases as the turbine speed increases to pull the blade into a tangential position with its orbit where it generates lift force.
Centrifugal force generated on a blade in its orbital movement has also been used as a means for positioning a blade. U.S. Pat. No. 4,048,947, Sicard shows a method of positioning a blade using centrifugal force where a weight is rigidly attached to a blade and extended obliquely outward therefrom to change the center of mass of the blade respond which the centrifugal force acts, thereby controlling the pitch angle of the blade as turbine speed increases.
Oscillatory pivotal motions of the blade have been employed which are effected by the wind current impacting on the surface of the blade, to direct the blade pitch angle to a more favorable position. The pivotal movement of the blade is generally controlled by a biasing means, such as a spring, which urges the blade into a tangential position with its orbit. The biasing means allows the blade to position itself in a state of balance between the force exerted on it in deflecting the wind current and the tension exerted on it by the biasing means. This type of control has been used to provide for self-starting the turbine, as well as maximizing its efficiency at a specific rotational speed and to limit the maximum rotational speed of the turbine by angular positioning of the blades, as is described in the U.S. patent of Rumsey, No. 4,052,134 and the Canadian patent of Cameron, No. 1,045,038.
Past designs have succeeded in providing a self-starting capability for lift-type wind turbines. They have further been able to provide articulating blade features which enhance efficiency and power at a specific turbine rotational speed, and which can limit the turbines maximum rotational speed to prevent damage from centrifugal forces in an over-speed condition. However, none of the previous designs have provided a wind turbine which is capable of self-starting while being highly efficient throughout its entire operational speed range.